JPH0529163B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a reference current source circuit used in a time constant circuit such as a filter circuit using built-in capacitance in a semiconductor integrated circuit.

(Prior art) Generally, a semiconductor integrated circuit (hereinafter referred to as IC)
When forming a circuit using built-in capacitance, it is necessary to adjust the reference current necessary for circuit operation in order to compensate for variations in built-in capacitance. Fourth
The figure shows the basic configuration of a conventional low-pass filter circuit that uses built-in capacitance in an IC.

In FIG. 4, transistors Q ₁ to _{Q 5} constitute a differential amplifier circuit. This differential amplifier circuit has Q ₁ ,
It supplies a current corresponding to the base potential difference of _Q2 to the IC built-in capacitor _C1 . The charging voltage of capacitor _C1 is outputted via buffer transistor _Q6 and is also fed back to the base of _Q2 . On the other hand, the base of Q ₁ is biased by the bias power supply voltage V _B , and the base potential changes according to the input voltage V _io with V _B as a reference. In addition, I ₀ in the figure
is a constant current source.

That is, this low-pass filter circuit obtains low-pass filter characteristics by changing the charging current of the built-in capacitor _C1 according to the voltage difference between the input voltage _Vio and the output power _Vput . The cutoff frequency _c of this circuit is determined by setting the collector current of transistor Q ₅ to i ₁ and the output gain of the differential amplifier circuit.
When g _n , it is expressed as _c = g _n /2πC ₁ ...(1). Here, g _n =i ₁ /2·V _T (V _T =KV/q) ...(2).

Here, as is clear from equations (1) and (2), variations in capacitor _C1 lead to variations in cutoff frequency _c . Therefore, in the circuit shown in Figure 4, the collector current _i1 of transistor _Q5 is controlled by transistor _Q7 , which forms a current mirror circuit with _Q5 , and an external variable resistor that adjusts the current of this current mirror circuit.
Provide a VR, control from the outside using the VR,
The cutoff frequency can be adjusted.

(Problem to be Solved by the Invention) However, in the conventional adjustment means as described above, adjustment is performed using an external variable resistor, so when assembling a set using such an IC, the time required for adjustment is Costs cannot be avoided due to losses and the need for external parts.

This invention was made to solve the above problem, and even if a time constant circuit is constructed using the built-in capacitance of an IC, the reference current that drives the time constant circuit cannot be adjusted in response to variations in the capacitance. It is an object of the present invention to provide a reference current source circuit that can be set and obtain desired time constant characteristics.

[Structure of the Invention] (Means for Solving the Problems) The reference current source circuit according to the present invention inputs a clock signal of a predetermined amplitude and alternately turns on and off according to the maximum and minimum values of the clock signal. and a first transistor that is charged and discharged by turning on and off the first and second transistors.
a second capacitor that averages the current flowing through at least one of the first and second transistors, and a third transistor that is driven by the current averaged by the second capacitor. The current flowing through the third transistor is taken out and used as a reference current.

(Function) The reference current source circuit with the above configuration constitutes a class B output circuit by the first and second transistors,
The first capacitor is charged and discharged via this class B output circuit in accordance with the input clock signal. Then, this charging/discharging current is averaged by a second capacitor, a third transistor is driven according to the average output, and the current flowing through the transistor is taken out and used as a reference current. For this reason, the first capacitor
If the IC has a built-in capacitance, a reference current proportional to the built-in capacitance can be obtained.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 shows a configuration in which the present invention is applied to the low-pass filter circuit shown in FIG. 4. In FIG. 1, the same parts as in FIG. 4 are designated by the same reference numerals.

In FIG. 1, _ECK is an external clock generation source, and the clock _VCK generated here is input to the base of transistor _Q11 inside the IC. This transistor Q ₁₁ forms a differential pair of transistors Q ₁₂ ,
Constant current source I ₁ , diode D ₁ and transistor Q ₁₃
The collector output of transistor _Q12 is supplied to the bases of transistors _Q14 and _Q15 . Transistors Q ₁₄ and Q ₁₅ are transistors
Q ₁₆ , Q ₁₇ and constant current source I ₂ constitute a class B output buffer circuit, the output terminal a of which is connected to a time constant circuit consisting of resistor R ₁ and built-in capacitor C ₂ , and transistor Q ₁₂ connected to the base of Further, the transistor _Q17 constitutes a current mirror circuit with the transistor _Q5 ,
Its common base is connected to an external capacitor _C3 . _C3 is for averaging the output current _i3 of _Q17 .

The operation of the above configuration will be described below with reference to FIG.

First, both the differential amplifier circuit and the output buffer circuit made up of the transistors Q ₁₁ to Q ₁₇ have a gain of 1.
Assuming that, a voltage equivalent to the input clock voltage _VCK is generated at point a in the figure, and the transistor
Q ₁₅ and Q ₁₆ are turned on alternately every half cycle of the clock, and the charging/discharging current of capacitor C ₂ flows.
This charging/discharging current is limited by resistor _R1 .
Therefore, if the minimum value of the input clock is V ₁ and the maximum value is V ₂ , and the maximum value of the output current i ₃ of Q ₁₆ is i _3nax , then i _3nax = V ₂ − V ₁ /R ₂ ...( 3) becomes. At this time, the relationship between the clock voltage _VCK and the current _i3 is shown in FIGS. 2a and 2b, respectively.

Here, the amount of charge Q that the capacitor _C2 charges and discharges
Q = (V ₁ − V ₂ )・C ₂ ...(4). Therefore, one cycle of the clock T _CK
The average value of the current _{i 3} flowing through the transistor Q ₁₆ at
₃ =Q/ _TCK =( _V1 - _V2 )・_C2 / _TCK =( _V1 - _V2 )・_C2・_CK ...(5). As is clear from equation (5), ₃ is proportional to the clock frequency _CK and the capacitor capacitance value _C2 .
This ₃ becomes the collector current i ₁ of the transistor Q ₅ , so if ₃ is used as the reference current, the cutoff frequency _c of the low-pass filter circuit is _c = g _n /2π・C ₁ = (V ₁ − V ₂ )・C ₂・_CK /2π・C ₁・V _T ...(6)

Therefore, as is clear from equation (6), if the clock frequency _c and (V ₁ −V ₂ ) are kept constant,
Since the capacitance ratio C ₂ /C ₁ inside the IC can be set accurately, the time constant of the low-pass filter circuit can be determined without adjustment.

FIG. 3 shows a modification of the circuit shown in FIG.
In FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals. That is, in this circuit, the collector current of the transistor _Q12 is taken out by a current mirror circuit consisting of a diode _D2 and a transistor _Q18 , and the mirror output is used to extract the collector current of the transistor Q12.
Drives Q ₁₅ and Q ₁₆ . And transistor Q ₁₅
The current flowing through the transistors Q ₁₉ and Q ₂₀ is taken out by a current mirror circuit, and the mirror output is supplied to the common base of the transistors Q ₁₇ and Q ₅ .

In this circuit, the collector current of transistor Q ₁₅ is folded back by transistors Q ₁₉ and Q ₂₀ and added to the common base of Q ₁₇ and Q ₅ , so it has the advantage that a current twice as large as that of the circuit shown in FIG. 1 can be obtained. Since the other operations are almost the same as those of the circuit shown in FIG. 1, their explanation will be omitted here.

[Effects of the Invention] As described above, according to the present invention, even if a time constant circuit is configured using a built-in capacitor in an IC, the reference current that drives the time constant circuit can be adjusted in response to variations in the capacitance without needing to be adjusted. It is possible to provide a reference current source circuit that can be set and obtain desired time constant characteristics.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the reference current source circuit according to the present invention, FIG. 2 is a waveform diagram for explaining the operation of the same embodiment, and FIG. 3 is another embodiment according to the present invention. FIG. 4 is a circuit diagram for explaining a reference current adjusting means for a conventional low-pass filter circuit with built-in IC. Q ₁₁ to Q ₁₇ ... Transistor, I ₁ , I ₂ ... Constant current source, R ₁ ... Current limiting resistor, C ₁ , C ₂ ... IC built-in capacitor, C ₃ ... External capacitor, E _CK ... ...external clock source.

[Claims]

1 Equipped with an auto preset function that searches within the reception band and selects a reception frequency above a certain level, and stores the reception frequency information of the selected station in a rewritable preset memory, and when selecting a station, it corresponds to the reception station. In a radio receiver configured to read the receiving frequency information of the corresponding receiving station from the preset memory by operating the tuning button to automatically set the tuning frequency of the tuning circuit to the corresponding receiving frequency, the reference input electric field strength is set. a field strength level setting circuit for setting a field strength level, and a field strength measurement control circuit for automatically instructing to measure the input field strength of the receiving station preset in the preset memory at a predetermined time interval over all stations; If there is a station whose input level is lower than the reference input electric field strength set in the field strength level setting circuit among the receiving stations that are being set, the auto preset function is activated and the preset station with the lower input level is selected. 1. An automatic channel selection device for a radio receiver, characterized in that a new station having an input level equal to or higher than a reference input electric field strength is automatically preset.